PREFACE. 



In the present volume I have brought together in one connected 

 presentation the chief results of my investigations cpncerning the 

 factors of organic evolution. Portions of my theory of divergence 

 which were pubKshed in the Ivinnean Society's Journal are repro- 

 duced in the Appendix, with careful revision; but the fullest- exposi- 

 tion of the fact that all evolution, as we now observe it, is divergent, 

 and that other factors besides natural selection are absolutely neces- 

 sary both for the origin and the continuance of this divergence, is 

 given in the new chapters constituting the body of the volume. 

 These chapters have been written while considering the most recent 

 biological investigations bearing on the general theory of segregation. 

 The first four chapters of the volume are introductory, in that they 

 present many facts of divergence and distribution in both natural and 

 domestic species, which remain complete enigmas till the forms of 

 racial and habitudinal segregation have been fully recognized. Chap- 

 ters V, VI, and VII present the fundamental laws of segregation, and 

 the interaction between the different classes of factors — between 

 isolation and selection, between racial segregation and habitudinal 

 segregation, between autonomic factors and heteronomic factors. 

 In Chapter VI, § II, i4-i7(pp. loi-iii), will be found a fuller exposi- 

 tion than has been presented in any of my essays published by the 

 I/innean Society, of the tendency of certain combinations of partially 

 segregative endowments to become more intense in successive genera- 

 tions. It is shown that this is especially the case when endowments, 

 tending toward the mating of like forms with each other, are reinforced 

 by varying degrees of mutual infertility and incompatibility between 

 unlike forms. Appendix II, § IV, 3 (pp. 241-243), briefly indicates 

 several methods of constructing what I have called the permutation 

 triangle. It was first constructed in order to show that the sterility of 

 cross-unions between divergent forms, (whether they be varieties, 

 species, genera, or higher groups), would lead rapidly to the extinction 

 of most of these forms, if instincts and other endowments did not 

 faciUtate the union of compatible forms. The table thus constructed 

 is found to be a concise presentation of certain classes of probabilities 

 that arise in the pairing of things by chance. 



